1. Field of the Invention
The present invention relates to a pressure detection apparatus, and more specifically to a pressure detection apparatus of a type wherein a semiconductor pressure sensor is interposed between the side of reference pressure of fluid, preferably, pressurized air, and the side of measured pressure thereof so as to obtain the difference between the pressure on the side of the reference pressure and that on the side of the measured pressure by driving the semiconductor pressure sensor at a constant voltage, and an offset value of an output fed from the semiconductor pressure sensor and varying with the external atmospheric temperature, is corrected under predetermined temperature, thereby enabling a feed system which makes use of pressure in fluid or the like to be easily and positively controlled.
Heretofore, feed apparatus which make use of air pressure have widely been employed in manufacturing factories or the like. This is because, for example, the apparatus is relatively risk free, is low in manufacturing costs, and is simple to maintain and control.
This type of feed apparatus is designed, for example, to use a suction pad, which is in turn connected to a vacuum source, so as to adsorb a work under action of negative pressure of the suction pad for feeding the same. At this time, if the negative pressure does not reach a predetermined value, the work cannot be adsorbed and carried at all. In addition, there is a potential problem that such apparatus may be damaged accidentally, such as by the work falling, if the vacuum level of the work is not kept at all times in a predetermined region. Therefore, a pressure detection mechanism has conventionally been incorporated into this type of apparatus. The prior art which is suitable for this type of mechanism, has been disclosed in, for example, Japanese Utility Model Application Laid-Open No. 62-9106. In other words, a pressure detection element, which makes use of piezoresistance effects and is driven by the current, is interposed between a pressure passage for comparison and a pressure passage for measurement. When a constant current is supplied to the pressure detection element, the comparison circuit serves to compare an analog signal produced corresponding to the difference in pressure referred to above with a separately established comparison reference voltage for generating an on or off two-position signal, thereby to convert the two-position signal into a control signal as control output by means of an output circuit. In other words, a comparison is made between an output voltage from the pressure detection element and the comparison reference voltage to obtain a control output.
Meanwhile, the semiconductor pressure sensor, which makes use of the piezoresistance effects, is superior to other capacitance type pressure sensors, metallic gauge type pressure sensors or mechanical pressure sensors in that the semiconductor pressure sensor can be fabricated at low cost, be mass-produced and has consistent sample-to-sample performance. However, on the other hand, the semiconductor pressure sensor will reveal the drawback that it is strongly dependent on temperature. Especially, when it is desired to perform a current drive of the semiconductor pressure sensor, the semiconductor pressure sensor making use of the piezoresistance effects shows a particularly dependency on temperature. As a result, where it is desired to incorporate this type of semiconductor pressure sensor into the pressure detection apparatus which has already been disclosed in Japanese Utility Model Application Laid-Open No. 62-9106, the difference between the comparison voltage and the measured voltage cannot be obtained in a stable state owing to the external atmospheric temperature. As a consequence, there exists a potential problem that the control output always remains unstable. Accordingly, where this type of pressure detection apparatus is employed in the feed system or the like, the control of the vacuum source must be performed more accurately. In addition, inconvenience such as complication of the construction of the control circuit takes place because of the constant-current drive.